The XXI is the granddaddy of them all.

In a way all modern submarines are derived from the XXI. The XXI was the first submarine, all other before were submersibles. It was the first designed for the concept of always staying submerged which was accomplished with the help of the snorkel. It was also the first to have underwater speed greater than surface speed due to big electric motors and big batteries. It also had a central air system with central CO2 removal, which allowed it to stay down without snorkeling for over 5 days. The American submarines of the time had the big motors, but lacked the big batteries. Further, it was the first to really use streamlining to get less drag. Obviously later submarines had different hull designs and improved motors and systems, but the basic concept is unchanged. Most Diesel submarines in the 1950s were close copies of the XXI and Nautilus had a very similar hull design. BTW, the figure 8 hull was far from ideal and would have been replaced as soon as the advantages of the modified tear drop shape were found.

Regards,
SuperKraut